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United States Patent |
5,534,386
|
Petersen
,   et al.
|
July 9, 1996
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Homogenizer formed using coherent light and a holographic diffuser
Abstract
A homogenizer for incident light including: a sheet of embossable material
including a one micro-sculpted surface relief structure that (i) controls
the direction in which light propagates and (ii) homogenizes light with
directionality has been formed by replicating in the sheet of embossable
material another micro-sculpted surface structure that (i) controls the
direction in which light propagates and (ii) homogenizes light with
directionality, the another micro-sculpted surface structure having been
formed in a photosensitive medium having a refractive index by: (a)
generating random, disordered and non-planar speckle in the photosensitive
medium using coherent light, the coherent light having been diffused
through a holographic diffuser, so as to define non-discontinuous and
smoothly varying changes in the refractive index of the photosensitive
medium, the smoothly varying changes scattering collimated light into a
controlled pattern with smooth brightness variation; and (b) developing
the photosensitive medium. Light that is incident on and directed from the
homogenizer is directed to an output area, the homogenizer controlling the
direction of light that is emanating from the homogenizer to the output
area so as to increase brightness in the output area relative to an area
outside the output area.
Inventors:
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Petersen; Joel (Valley Village, CA);
Lerner; Jeremy (Culver City, CA)
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Assignee:
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Physical Optics Corporation (Torrance, CA)
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Appl. No.:
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393050 |
Filed:
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February 23, 1995 |
Current U.S. Class: |
430/320; 264/1.31; 264/1.9; 264/2.5; 359/15; 359/22; 359/599; 430/1; 430/2; 430/321 |
Intern'l Class: |
G03H 001/32; G02D 051/32 |
Field of Search: |
430/1,2,4,320,321,290
264/1.2,14,1.9,2.5
359/599,15,22
362/335
|
References Cited
U.S. Patent Documents
3708217 | Jan., 1973 | McMahon | 350/3.
|
4006965 | Feb., 1977 | Takada et al. | 350/117.
|
4040717 | Aug., 1977 | Cinque et al. | 350/127.
|
4053208 | Oct., 1977 | Kato et al. | 362/355.
|
4206969 | Jun., 1980 | Cobb et al. | 359/452.
|
4268118 | May., 1981 | Palmquist et al. | 350/128.
|
4290696 | Sep., 1981 | Mould et al. | 350/162.
|
4309093 | Jan., 1982 | Kuwayama et al. | 354/59.
|
4336978 | Jun., 1982 | Suzuki | 359/599.
|
4427265 | Jan., 1984 | Suzuki et al. | 350/321.
|
4428648 | Jan., 1984 | Wiley | 350/238.
|
4481414 | Nov., 1984 | Gasper | 359/634.
|
4523807 | Jun., 1985 | Suzuki | 350/128.
|
4545646 | Oct., 1985 | Chern et al. | 350/167.
|
4558922 | Dec., 1985 | Smith | 350/127.
|
4567123 | Jan., 1986 | Ohtaka et al. | 359/615.
|
4888201 | Dec., 1989 | Veenvliet et al. | 264/1.
|
4968117 | Nov., 1990 | Chern et al. | 350/164.
|
5046793 | Sep., 1991 | Hockley et al. | 359/599.
|
5048925 | Sep., 1991 | Gerritsen et al. | 359/569.
|
5300263 | Apr., 1994 | Hoopman et al. | 264/1.
|
5365354 | Nov., 1994 | Jannson et al. | 359/15.
|
Foreign Patent Documents |
479490 | Apr., 1992 | EP | 359/15.
|
53-42726 | Apr., 1978 | JP.
| |
61-86221 | May., 1986 | JP | 264/2.
|
62-12939 | Jan., 1987 | JP.
| |
Other References
Dialog, JPO & JAPIO Database, Accession No. 00240726, (i.e., record
including an English language abstract for JP 53042726 A), dated June 17,
1978.
Cowan, J. J. "Blazed Holographic Gratings . . . " SPIE vol. 240 Proc. Joc.
Photo-opt Instrum. Eng. 1980 pp. 5-12.
|
Primary Examiner: Bowers, Jr.; Charles L.
Assistant Examiner: Angebranndt; Martin J.
Attorney, Agent or Firm: Nilles & Nilles
Parent Case Text
This application is a continuation of application Ser. No. 08/097,953,
filed Jul. 27, 1993, now abandoned.
Claims
We claim:
1. A process for making a homogenizer comprising:
A. generating random disordered non-planar speckle in a photosensitive
medium having a refractive index by exposing said photosensitive medium
with coherent light, said coherent light having been diffused through a
holographic diffuser, so as to define non-discontinuous and smoothly
varying changes in said refractive index of said photosensitive medium,
said non-discontinuous and smoothly varying changes scattering collimated
light into a controlled pattern with smooth brightness variation;
B. developing said photosensitive medium to form a micro-sculpted surface
structure that (i) controls the direction in which light propagates and
(ii) homogenizes light with directionality; and
C. making a replica of said micro-sculpted surface structure formed in said
photosensitive medium,
wherein light scattered from said replica of said micro-sculpted surface
structure is characterized by said random, disordered and non-planar
speckle.
2. The process as defined in claim 1, wherein said holographic diffuser is
a master holographic diffuser.
3. The process as defined in claim 1, further comprising:
placing a release agent on said micro-sculpted surface structure so as to
form a release coating; and
making said replica by coating a substrate with an epoxy, placing said
epoxy against said release coating, curing said epoxy, and separating said
epoxy from said release coating.
4. The process as defined in claim 3, wherein said substrate is selected
from the group consisting of a glass substrate, a plastic substrate and a
metal substrate.
5. The process as defined in claim 3, further comprising making a metal
master from said replica using an electroform process.
6. The process as defined in claim 5, further comprising:
producing said homogenizer from said metal master by thermoplastic
embossing a thermoplastic material with said metal master.
7. A product produced by the process defined by claim 1.
8. A homogenizer having a normal axis, a horizontal direction, and a
vertical direction, said homogenizer comprising:
A. a sheet of deformable material having formed therein a one
micro-sculpted surface structure that (i) controls the direction in which
light propagates and (ii) homogenizes light with directionality, said one
micro-sculpted surface structure having been formed by replicating in said
sheet of deformable material another micro-sculpted surface structure that
(i) controls the direction in which light propagates and (ii) homogenizes
light with directionality, said another micro-sculpted surface structure
having been formed in a photosensitive medium having a refractive index
by:
(a) generating random, disordered and non-planar speckle in said
photosensitive medium using coherent light, said coherent light having
been diffused through a holographic diffuser, so as to define
non-discontinuous and smoothly varying changes in said refractive index of
said photosensitive medium, said non-discontinuous and smoothly varying
changes scattering collimated light into a controlled pattern with smooth
brightness variation; and
(b) developing said photosensitive medium; and
B. a reflective layer deposited on and conforming to said one
micro-sculpted surface structure, whereby light incident on said
homogenizer is reflected into an output area by said one micro-sculpted
surface structure,
wherein light scattered from said one micro-sculpted surface structure is
characterized by said random, disordered and non-planar speckle.
9. The homogenizer as defined in claim 8, wherein said output area has an
output area normal axis which is off the normal axis of said homogenizer.
10. The homogenizer as defined in claim 8, wherein said one micro-sculpted
surface structure is made by recording said another micro-sculpted surface
structure in said photosensitive medium, making a replica of said another
micro-sculpted surface structure from said photosensitive medium, making a
metal master of said replica from said replica, and embossing said sheet
of deformable material with said metal master.
11. The homogenizer as defined in claim 10, wherein said photosensitive
medium includes dichromated gelatin.
12. The homogenizer as defined in claim 10, wherein said holographic
diffuser is a master holographic diffuser.
13. A homogenizer for incident light comprising:
a sheet of embossable material including a one micro-sculpted surface
relief structure that (i) controls the direction in which light propagates
and (ii) homogenizes light with directionality has been formed by
replicating in said sheet of embossable material another micro-sculpted
surface structure that (i) controls the direction in which light
propagates and (ii) homogenizes light with directionality, said another
micro-sculpted surface structure having been formed in a photosensitive
medium having a refractive index by:
(a) generating random, disordered and non-planar speckle in said
photosensitive medium using coherent light, said coherent light having
been diffused through a holographic diffuser, so as to define
non-discontinuous and smoothly varying changes in said refractive index of
said photosensitive medium, said smoothly varying changes scattering
collimated light into a controlled pattern with smooth brightness
variation; and
(b) developing said photosensitive medium,
wherein light that is incident on and directed from said homogenizer is
directed to an output area, said homogenizer controlling the direction of
light that is emanating from said homogenizer to said output area so as to
increase brightness in said output area relative to an area outside said
output area and light scattered from said one micro-sculpted surface
structure is characterized by said random, disordered and non-planar
speckle.
14. The homogenizer as defined in claim 13, wherein said one micro-sculpted
surface relief structure is characterized by peaks and valleys in a
surface of said homogenizer so as to yield an output that is selected from
the group consisting of circular, elliptical, and rectangular.
15. The homogenizer as defined in claim 14, wherein said output area is
elliptical and said peaks and valleys are substantially straight and of
random length.
16. The homogenizer as defined in claim 14, wherein said output area is
rectangular and said one micro-sculpted surface relief structure includes
two intersecting sets of peaks and valleys.
17. The homogenizer as defined in claim 14, having an output intensity
which is constant over a field of view.
18. A process for making a homogenizer comprising:
A. generating random disordered non-planar speckle in a photosensitive
medium having a refractive index by exposing said photosensitive medium
with coherent light, said coherent light having been diffused through a
master holographic diffuser, so as to define non-discontinuous and
smoothly varying changes in said refractive index of said photosensitive
medium, said non-discontinuous and smoothly varying changes scattering
collimated light into a controlled pattern with smooth brightness
variation;
B. developing said photosensitive medium to form a micro-sculpted surface
structure that (i) controls the direction in which light propagates and
(ii) homogenizes light with directionality;
C. placing a release agent on said micro-sculpted surface structure so as
to form a release coating; and
D. making a replica of said micro-sculpted surface structure formed in said
photosensitive medium by coating a substrate with an epoxy, placing said
epoxy against said release coating, curing said epoxy, and separating said
epoxy from said release coating,
wherein light scattered from said replica of said micro-sculpted surface
structure is characterized by said random, disordered and non-planar
speckle.
19. The process as defined in claim 18, further comprising making a metal
master from said replica using an electroform process.
20. A homogenizer having a normal axis, a horizontal direction, and a
vertical direction, said homogenizer comprising:
A. a sheet of deformable material having formed therein a one
micro-sculpted surface structure that (i) controls the direction in which
light propagates and (ii) homogenizes light with directionality, said one
micro-sculpted surface structure having been formed by replicating in said
sheet of deformable material another micro-sculpted surface structure that
(i) controls the direction in which light propagates and (ii) homogenizes
light with directionality, said another micro-sculpted surface structure
having been formed in a photosensitive medium having a refractive index
by:
(a) generating random, disordered and non-planar speckle in said
photosensitive medium using coherent light, said coherent light having
been diffused through a holographic diffuser, so as to define
non-discontinuous and smoothly varying changes in said refractive index of
said photosensitive medium, said non-discontinuous and smoothly varying
changes scattering collimated light into a controlled pattern with smooth
brightness variation; and
(b) developing said photosensitive medium; and
B. a reflective layer deposited on and conforming to said one
micro-sculpted surface structure, whereby light incident on said
homogenizer is reflected into an output area by said one micro-sculpted
surface structure, said output area having an output area normal axis
which is off the normal axis of said homogenizer,
wherein said one micro-sculpted surface relief structure is characterized
by peaks and valleys in a surface of said homogenizer so as to yield an
output that is selected from the group consisting of circular, elliptical,
and rectangular and light scattered from said one micro-sculpted surface
structure is characterized by said random, disordered and non-planar
speckle.
21. The homogenizer as defined in claim 20, wherein said output area is
elliptical and said peaks and valleys are substantially straight and of
random length.
22. A homogenizer for incident light comprising:
a sheet of embossable material including a one micro-sculpted surface
relief structure that (i) controls the direction in which light propagates
and (ii) homogenizes light with directionality has been formed by
replicating in said sheet of embossable material another micro-sculpted
surface structure that (i) controls the direction in which light
propagates and (ii) homogenizes light with directionality, said another
micro-sculpted surface structure having been formed in a photosensitive
medium having a refractive index by:
(a) generating random, disordered and non-planar speckle in said
photosensitive medium using coherent light, said coherent light having
been diffused through a holographic diffuser, so as to define
non-discontinuous and smoothly varying changes in said refractive index of
said photosensitive medium, said smoothly varying changes scattering
collimated light into a controlled pattern with smooth brightness
variation; and
(b) developing said photosensitive medium,
wherein 1) light that is incident on and directed from said homogenizer is
directed to an elliptical output area, 2) said homogenizer controls the
direction of light that is emanating from said homogenizer to said output
area so as to increase brightness in said output area relative to an area
outside said output, 3) said one micro-sculpted surface relief structure
is characterized by peaks and valleys in a surface of said homogenizer
that are substantially straight and of random length having an output
intensity which is constant over a field of view area and 4) light
scattered from said one micro-sculpted surface structure is characterized
by said random, disordered and nonplanar speckle.
23. The homogenizer as defined in claim 22, wherein said output area has an
output area normal axis which is off a normal axis of said homogenizer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to homogenization of light sources. More
particularly, this invention pertains to a homogenizer which destructures
and shapes light.
2. Description of the Prior Art
Prior art diffusers or homogenizers scatter light in various directions,
and the intensity in a particular direction depends on the diffuser
structure. There are numerous types of conventional diffusers such as
ground glass, opal glass, opaque plastics, chemically etched plastics, and
machined plastics. Cloth and nylon diffusers are used as well. All of
these prior art diffusers have shortcomings that make them unsuitable for
many applications. Transmission efficiency is poor, and it is not possible
to control the direction, or shape, of diffused light with most of these
diffusers.
Lenticular, or machined plastic diffusers, can be made to control the angle
of diffused light by varying the characteristics of the surface structures
on the diffuser. In this sense, lenticular diffusers are more capable than
most other conventional diffusers because, at least, the angle of diffused
light can be partially controlled. Nonetheless, lenticular diffusers are
undesirable in many applications because they are extremely complex
macro-sized, two-sided structures. This makes them difficult and expensive
to produce and not well adapted for very high resolution applications.
Also, they generate significant side lobes which means that, even if the
angle of diffused light can be controlled, much of the light energy is
lost in the side lobes and is not transmitted through the desired
aperture. Consequently, brightness suffers and higher intensity sources
must be used to compensate for these losses.
SUMMARY OF THE INVENTION
A light source destructuring and shaping device is presented. More
specifically, a device which both homogenizes and imparts predetermined
directionality to light rays emanating from a light source is disclosed.
The device of the present invention comprises a micro-sculpted surface
structure which controls the direction in which light propagates in either
reflection or transmission. The sculpted surface structure also
homogenizes light propagating through it with predetermined
directionality. The device may be used in an almost unlimited number of
applications which require homogenization and which would benefit from
being able to impart directionality to light waves emanating from a light
source. These applications include existing diffuser applications and
applications wherein it was not practical to use conventional diffusers.
The homogenizing and shaping device of the present invention achieves
these benefits with very high transmission or reflection efficiency and
with reduced side lobes.
A method by which the device of the present invention is made comprises the
steps of generating a surface structure in a photosensitive medium using
coherent light, processing the medium, and replicating the surface
structure in, for example, epoxy. The surface structure may be generated
in the photosensitive medium by exposing it to coherent light which has
been diffused. The light may be diffused by a ground glass, holographic,
lenticular, or acetate diffuser, for example. The photosensitive medium
may comprise, for example, dichromated gelatin, photoresist, silver
halide, or photopolymer. Once the photosensitive medium is recorded and
processed, any of a number of types of epoxy, or its equivalent, may be
applied thereto to transfer the surface structure into the epoxy, which,
when cured, may be separated from the medium. The cured epoxy layer may be
used, as is, in a transmission application, or coated with a reflective
material for a reflection application. For mass production, the epoxy
layer may be subjected to electroform processes, or its equivalent, to
create a metal master from which plastic, or other embossable materials,
may be imprinted with the sculpted surface structure.
The surface structure of the device of the present invention controls
directionality of light waves emanating from a light source in such a way
that light may be directed into well defined fields of view. Furthermore,
brightness or gain (number of photons per unit area) in this field of view
is significantly increased because of the highly efficient surface
structure of the device and because the light is not directed to areas
where it is not desired. The applications of the device of the present
invention are virtually unlimited,
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of a predefined output area of a homogenizer of the
present invention;
FIGS. 2A and B depict the homogenizer of the present invention
destructuring light from a source;
FIG. 3 depicts the homogenizer of the present invention providing
directional lighting in the workplace;
FIG. 4 depicts the homogenizer of the present invention shaping light
incident a piece of art;
FIG. 5A is a recording set-up using an objective lens and a transmissive
diffusing material used to record the photosensitive medium and FIG. 5B is
a photograph of the surface of a 20.degree. circular homogenizer magnified
several hundred times;
FIG. 6 is a recording set-up using an objective lens and a reflective
diffusing material to record the photosensitive medium;
FIG. 7 is a recording set-up using two lenses and a transmissive diffusing
material to record the photosensitive medium;
FIGS. 8A and B are schematics of light passing through a cylindrical lens;
FIG. 9A depicts light incident on a master diffuser from a cylindrical
lens, FIG. 9B depicts speckle recorded in a photosensitive medium using a
cylindrical lens, FIG. 9C depicts the angular output of a homogenizer of
the present invention; FIG. 9D depicts speckle recorded in a
photosensitive medium, FIG. 9E depicts the angular output of a homogenizer
of the present invention, and FIG. 9F is a photograph of the surface of a
homogenizer of the present invention;
FIG. 10 is a recording set-up using an objective lens and two holographic
diffusers to record the photosensitive medium;
FIGS. 11A and B depict the angular output of a homogenizer of the present
invention recorded sequentially with elongated elliptical speckle in one
direction and elongated elliptical speckle in a perpendicular direction;
FIG. 11C is a photograph of the surface of a homogenizer of the present
invention depicting the output of such a homogenizer;
FIGS. 12A-E compare the FWHM (full width half maximum) of a 20.degree.
homogenizer of the present invention (12A), a 10.degree. homogenizer of
the present invention (12B), with a 20 micron ground glass diffuser (12C),
an acetate diffuser (12D), and a lenticular diffuser (12E).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, a homogenizer 2 of the present invention is shown
having an exemplary predefined output area 3. Homogenizer 2 may comprise
any number of embossable materials such as plastic containing a sculpted
surface structure suitable for directing light incident thereon into a
predefined output area 3. "Directed" light in this sense includes both
light reflected from a reflective homogenizer and light transmitted
through a transmissive homogenizer into a predefined output area. The
homogenizer 2 may be any size or shape desired and would depend on the
confines of the area in which it is to be used. A transmissive homogenizer
of the present invention may simply comprise a sheet of plastic or other
embossable translucent material which has been embossed with a sculpted
surface structure suitable for transmitting light into the predefined
output area 3 or a glass substrate with an epoxy replication layer on it.
As seen in FIG. 1, the exemplary predefined output area is a rectangle.
Light is output within the predefined output area while only light at far
reduced intensity levels exists outside the predefined output area 3. The
surface structure in homogenizer 2 redirects light rays passing through
the homogenizer 2 into the predefined output area 3 and does so highly
efficiently. Light is actually redirected in the predefined output area so
that light intensity within that area is higher than it would be without
the homogenizer of the present invention. In other words, brightness is
significantly increased in the predefined output area 3.
In the case of a reflection homogenizer 2, a reflective coating such as
aluminum is deposited on the surface of the homogenizer 2 to reflect light
rays incident thereon in accordance with the sculpted surface structure.
Light is not directed into areas where it is not desired and instead is
redirected into the predefined output area increasing brightness in that
entire area.
The homogenizers of the present invention may be employed in a multitude of
applications. For instance, the homogenizer of the present invention has a
significant use as a light source destructuring device. In many
applications, it is desirable to eliminate from the output of the light
source itself the structure of the filament which can be problematic in
certain applications because light distributed across the sample will vary
and this is undesirable. Also, variances in the orientation of a light
source filament or arc after a light source is replaced can generate
erroneous and misleading readings. A homogenizer 4 of the present
invention placed between the light source 5 and the detector as seen in
FIG. 2A can eliminate from the output of the light source any trace of the
filament structure and therefore causes a homogenized output HO which is
identical from light source to light source.
Fiberoptic light assemblies mounted on a surgeon's headpiece can cast
distracting intensity variations on the surgical field if one of the
fiberoptic elements breaks during surgery. As seen in FIG. 2B, homogenizer
6 of the present invention placed at the ends of the fiber bundle 7
homogenizes light coming from the remaining fibers and eliminates any
trace of the broken fiber from the light cast on the patient. A standard
ground glass diffuser would not be as effective in this use due to
significant backscatter causing loss of throughput. In addition to
homogenizing, the homogenizer of the present invention may contain a
sculpted surface structure which not only homogenizes but directs light
into a wide field of view particularly suitable for surgery. The
homogenizer of the present invention may also be used in fiber optic
endoscope applications whereby the homogenizer may be placed at the end of
the endoscope optic to change the numerical aperture of the optic to match
that of the camera which displays the images from the body.
Scientific instruments analyze different media, such as biologicals,
organic and inorganic chemicals, by using methods such as emission and
absorption, fluorescence and Raman. In these analyses, the wavelength of
light, .lambda..sub.0, from the sample is undesirable and should be kept
away from the detector. The .lambda..sub.0 wavelength is typically avoided
by painting the inside of the device black to help absorb the light. This
is insufficient, however, because paint usually fluoresces and is
directional in its reflection and is never totally absorbent. A
homogenizer of the present invention adhered to the walls of the detector
area can redirect the .lambda..sub.0 wavelength away from the detector by
using an appropriate surface structure on the homogenizer. This type of
"light trap" is also useful in devices such as spectrometers to redirect
stray light.
The homogenizers of the present invention can also be used to homogeneously
illuminate a sample under a microscope by destructuring the filament or
arc of the source, yielding a homogeneously illuminated field of view. The
homogenizer may also be used to homogenize the various modes that
propagate through a fiber, for example, the light output from a
helical-mode fiber.
The homogenizers of the present invention also have significant
architectural uses such as providing appropriate light for work and living
spaces. In typical commercial applications, inexpensive plastic sheets
with a moulded macroscopic surface structure are used to help diffuse
light over the room. A homogenizer of the present invention which replaces
one of these conventional diffusers provides a more uniform light output
so that light is diffused to all angles across the room evenly and with no
hot spots. Furthermore, as seen in FIG. 3, the surface structure of the
homogenizer 8 may be made to direct light to a certain portion of the
room, such as a work area 9, where it is desired. This is done not by
absorbing light before it can reach the areas that are not to be lit but
by redirecting that light toward the areas desired to be lit.
The homogenizers of the present invention may also be used to diffuse light
illuminating artwork, as seen in FIG. 4. Homogenizer 10 on light source 11
provides a suitable appropriately sized and directed aperture 12 for
depicting the artwork 14 in a most desirable fashion.
The homogenizers may also be used to control lighting for stages by
providing pleasing homogenized light that is directed where desired. In
stage and television productions, a wide variety of stage lights must be
used to achieve all the different effects necessary for proper lighting.
This requires that many different lamps be used which is inconvenient and
expensive. The homogenizer of the present invention placed over a lamp can
give almost unlimited flexibility dispersing light where it is needed. As
a consequence, almost any object, moving or not, and of any shape, can be
correctly illuminated.
The homogenizers of the present invention may also be used in the area of
law enforcement and security systems to homogenize the output from laser
diodes (LDs) or light emitting diodes (LEDs) over the entire secured area
to provide higher contrasts to infrared (IR) detectors. The homogenizers
of the present invention may also be used to remove structure from devices
using LED or LD sources such as in bank note readers or skin treatment
devices. This leads to greater accuracy.
The homogenizers of the present invention may also be used in liquid
crystal display (LCD) display backlights where there is a fluorescent lamp
behind the LCD material. The homogenizer may be placed in front of the LCD
material in a transmission mode to disburse the light and make it much
more homogenous. The homogenizer of the present invention may also be
placed behind the fluorescent light source, in reflection, to homogenize
the light coming toward the viewer.
The preferred methods for making the directional homogenizers of the
present invention are now described. Generally, the first step is to
create a master diffuser, the second step is to record in a photosensitive
medium with coherent light passed through the master diffuser, and the
third step is to replicate the surface structure of the photosensitive
medium with, for example, epoxy. A fourth and optional step is to make a
metal electroform master from the epoxy for mass production purposes. In
the alternative, an electroform master may be made directly from the
master diffuser.
Referring to FIG. 5A, a recording set-up 16 is shown comprising a coherent
laser light source 18, objective lens 20, master diffuser 22, and
photosensitive medium 24. Coherent laser light source 18 is standard. The
objective lens 20 is standard and may be a low or high magnification lens
depending upon the desired characteristics of the photosensitive medium
24. The objective lens is spaced a distance X from the master diffuser 22.
The master diffuser 22 may comprise a standard ground glass diffuser, a
lenticular diffuser, an acetate diffuser, or a holographic diffuser. The
ground glass, lenticular, and acetate diffusers are conventional and made
in a conventional manner. If a holographic master diffuser is desired to
be used, that master diffuser may itself first be recorded in the
recording set-up shown in FIG. 5A with the holographic master diffuser to
be recorded being positioned at 24 and a conventional ground glass
diffuser being located at 22. That master diffuser may then be used to
record into another photosensitive medium to be used as a homogenizer of
the present invention.
A related set-up for recording volume holographic diffusers is described in
co-pending application Ser. No. 848,703, filed Mar. 9, 1992, now U.S. Pat.
No. 5,365,354, having a common assignee, the essentials of which are
incorporated herein by reference. According to that disclosure, recording
a holographic plate with coherent laser light passed through a
conventional ground glass diffuser generates features called speckle in
the volume of the hologram. This speckle is random, disordered and
non-planar speckle that defines non-discontinuous and smoothly varying
changes in the refractive index of the medium which scatter collimated
light into a controlled pattern with smooth brightness variation. The
size, shape, and orientation of the speckle can be adjusted which in turn
affects the angular spread of light scattered from the holographic
diffuser upon playback. Generally, the size of the angular spread of the
scattered light, in other words, the angular distribution of the scattered
light, depends on the average size and shape of the speckle. If the
speckle are small, angular distribution will be broad. If the speckle size
is horizontally elliptical, the shape of the angular distribution will be
vertically elliptical. Thus, it is desirable to control the size and shape
of speckle recorded in the medium so that, upon playback, the correct
output or angular spread is produced.
Speckle size is inversely proportional to the size of the aperture of the
master diffuser. If the size of the aperture increases, the size of the
speckle decreases and the size of the angular spread of the scattered
light from the recorded photosensitive medium increases. Conversely, if
the size of the master diffuser aperture decreases, the size of the
speckle recorded in the diffuser increases and the angular spread of light
scattered from the recorded photosensitive medium decreases. Thus, if the
master diffuser aperture is long and narrow, the speckle will be long and
narrow as well with their axes oriented perpendicularly to the axis of the
aperture. This holds true for both volume holographic recording media as
well as surface holographic recording media.
Diffusers made from volume holographic recording media as in Ser. No.
848,703, however, are disclosed there for their volume effect. In other
words, the speckle recorded in the interior or volume of the medium was
thought the only desired effect to be obtained from the material. However,
since then we have discovered that recording a volume holographic angular
spread such as DCG (dichromated gelatin) in a similar recording set-up
produces a surface effect of peaks and valleys which may be replicated as
described below.
The size, shape, and orientation of the surface features recorded in
photosensitive medium 24 is a function of a number of variables including
the type of objective lens 20 and master diffuser 22 used, as well as the
relative positioning of those components with respect to each other and
with respect to the photosensitive medium 24. Ultimately, the desired
results are obtained through empirical testing. In order to achieve a
recorded photosensitive medium having a particular surface structure that
can be replicated and comprise a homogenizer of the present invention, it
may be necessary to adjust the parameters discussed below to achieve the
desired shape of the light output.
The objective lens 20 expands the coherent laser light source 18 so that
the area of incidence (or "apparent aperture") of light from the objective
lens 20 on the master diffuser 22 is larger than that of the cross section
of the laser beam itself. The light beam expands in accordance with the
magnification of the objective lens 20.
Consequently, if a small magnification objective lens is used, such as 5X,
the aperture of light incident the master diffuser 22 will be smaller than
with a large magnification objective lens, such as 60X or greater, and
therefore the size of the surface features recorded in the photosensitive
medium 24 will be larger; the size of the aperture of light incident the
master diffuser 22 is inversely related to the size of the surface
features recorded in the photosensitive medium 24.
The distance between the objective lens 20 and the master diffuser 22 must
also be taken into account in achieving the desired sculpted surface
structure recorded in the photosensitive medium 24. As the distance
between the objective lens 20 and the master diffuser 22 decreases, i.e.,
as X decreases, the size of the speckle increases. This occurs because as
the objective lens 20 moves closer to the master diffuser 22, the apparent
aperture of light incident the master diffuser 22 is smaller. Because the
size of the speckle recorded in the photosensitive medium 24 is inversely
related to the size of the apparent aperture on the master diffuser 22,
the speckle will be larger. In turn, the increased speckle size recorded
in the photosensitive medium 24 will result in a homogenizer which has
decreased diffusion.
Conversely, if the distance X is increased, the apparent aperture of light
incident the master diffuser 22 will increase, thus decreasing the size of
the speckle recorded in the photosensitive medium 24 and in turn
increasing the amount of angular spread of the homogenizer.
The distance Y between the master diffuser 22 and the photosensitive medium
24 also affects speckle size. As the distance Y decreases, the size of the
speckle recorded in the photosensitive medium 24 decreases as well. This
occurs because, assuming an expanded beam of light is produced at
objective lens 20, as the photosensitive medium 24 is moved closer to the
master diffuser 22, the light beam emanating from each of the
irregularities in the master diffuser 22 will expand less by the time it
reaches the photosensitive medium 24, thus producing smaller speckle.
Conversely, if the distance Y is increased, the size of the speckle
recorded will be increased. Thus, these simple relationships between the
distances X, Y, and the magnification of the objective lens 20, are all
adjusted, empirically, to achieve the size of speckle desired in the
photosensitive medium 24.
Predefined output areas that are "off-axis" with respect to the normal axis
of the diffuser are achieved by tilting the photosensitive medium 24
around an axis normal to its surface. For example, a 20.degree. off axis
diffuser may be achieved by fitting the photosensitive medium 24 roughly
20.degree..
Assuming that a ground glass diffuser is used as the master diffuser 22,
the shape of the speckle recorded in photosensitive medium 24 will be
roughly round as will the shape of the angular output of a homogenizer
made from photosensitive medium 24. FIG. 5B is a photograph of the surface
of a homogenizer having a round angular output. A round output may also be
achieved when a lenticular or an acetate sheet is used as a master
diffuser 22. Lenticular sheets have tiny lens-like elements machined in
them. Acetate diffusers are made by an extrusion and embossing process
which also yields roughly round speckle. It is difficult to create or
control the desired irregularities in acetate diffusers. With respect to
lenticular diffusers, the surface effects necessary to achieve varying
output shapes are complex machined macroscopic structures. If a
prerecorded holographic master diffuser is used as the master diffuser 22,
additional degrees of recording freedom are achieved because the master
diffuser can be prerecorded with speckle having virtually any shape, size,
and orientation as discussed further below. Speckle characteristics are
more easily controlled using a holographic master diffuser.
In any case, in the recording set-up in FIG. 5A, the master diffuser must
be able to transmit light so that it reaches the photosensitive medium 24
from the objective lens 20. Thus, if a substrate is needed as part of the
master diffuser 22, such as if a holographic master diffuser is used, the
substrate should be capable of efficiently transmitting light. A glass
substrate is preferable. In addition to the additional degrees of freedom
which can be achieved by using a prerecorded volume or surface hologram as
the master diffuser 22, holographic master diffusers are preferable
because better uniformity of intensity in the photosensitive medium 24 is
achieved, higher transmission efficiency through the master diffuser 22 is
achieved, and the holographic master diffuser 22 causes less back scatter
than a ground glass diffuser. A first generation holographic volume master
diffuser may be made using a ground glass or acetate diffuser. This
holographic diffuser can then be used to make a second generation
holographic master diffuser, either volume or surface with greater
control.
Referring now to FIG. 6, a reflection recording set-up for recording in a
photosensitive medium 24 is depicted. Coherent laser light from light
source 18 is incident the objective lens 20 which collimates and expands
the light which is then incident upon reflective master diffuser 26
situated distance X from the objective lens 20. The light reflected from
the reflective master diffuser 26 is then incident upon the photosensitive
medium 24. The coherent laser light source 18, objective lens 20, and
photosensitive medium 24 retain the same numerals as in FIG. 5A because
they are the same elements. As in FIG. 5A, ground glass, lenticular,
acetate, or volume holographic master diffusers may be used but with the
addition of a suitably front reflective surface so that light is not
transmitted through master diffuser 26 but is reflected therefrom onto the
photosensitive medium 24. Variations in the distance X, distance Y, and
the magnification of the objective lens 20 have the same effect as
described above with respect to the recording set-up in FIG. 5A.
The distinction between the homogenizer of the present invention and
conventional diffusers is highlighted further when one considers the
ability to record surface features in the angular spread which are not
only round and produce round outputs as is conventionally found in ground
glass, acetate, and lenticular diffusers, but which produce surface
features, and therefore angular outputs, of any number of shapes not
possible before, including off-axis outputs.
Referring now to FIG. 7, a recording set-up using an additional lens 28
with the coherent laser light source 18, objective lens 20, master
diffuser 22, and photosensitive medium 24 is depicted in a transmission
recording set-up. As can be appreciated, many different types of lenses
can be used to shape the light beam from the coherent laser light source
18 before it reaches the master diffuser 22. Because one of the primary
objectives of the present invention is to achieve the desired sculpted
surface structure in photosensitive medium 24 which will yield the desired
angular spread, additional lens 28, which is positioned between the
objective lens 20 and the master diffuser 22, may be chosen to produce the
desired shape and orientation. In this case, additional lens 28 is a
cylindrical lens which outputs diverging light rays in one direction as
shown in FIG. 8A and light rays that converge to a line in another
direction as shown in FIG. 8B. Thus, the light rays that are incident the
master diffuser 22 in FIG. 7 are diverging with respect to each other in
one direction and converging upon each other into a line in the
perpendicular direction. Therefore, light rays passing through and exiting
the master diffuser 22 are likewise diverging more rapidly in a direction
perpendicular to the line of light on the master diffuser than are the
light rays that are parallel to that line.
In the recording set-up of FIG. 7, the master diffuser may preferably be
near or at the focal point of the cylindrical lens 28. If the master
diffuser 22 is at the focal point of the cylindrical lens 28, the maximum
effect from the cylindrical lens will be achieved. That effect can be
likened to stretching the speckle which are recorded in the photosensitive
medium 24 in one direction. As a result, the speckle recorded in
photosensitive medium 24 in the recording set-up in FIG. 7 will be long in
one direction and short in the perpendicular direction taking on roughly
the shape of the "line" shaped light beams produced by cylindrical lens
28, but oriented at 90.degree. thereto. FIG. 9A shows the light incident
on the master diffuser 22 from the cylindrical lens 28, which is aligned
in a horizontal direction. The speckle recorded in the photosensitive
medium 24 will have an orientation 90.degree. to this horizontal line as
seen in FIG. 9B and produce a narrow, long angular output as shown in FIG.
9C. If the master diffuser 22 is at the focal point of the cylindrical
lens 28, the degree of stretch of the speckle shown in FIG. 9B will be at
its maximum. If the master diffuser 22 is placed on either side of the
focal point of the lens 28, the speckle will tend to be shorter in the
vertical direction and wider in the horizontal direction as seen in FIG.
9D and produce a slightly wider, shorter angular output as shown in FIG.
9E. FIG. 9F is a photograph of the surface of such a homogenizer magnified
several hundred times. The elongated surface features, which appear as
peaks and valleys, are visible.
As also seen in FIG. 7, the objective lens 20 and cylindrical lens 28 are
separated by the distance X, the cylindrical lens 28 and the master
diffuser 22 are separated by the distance Y, and the master diffuser 22
and the photosensitive medium 24 are separated by the distance Z. As in
the above recording set-ups, if X is increased, the size of the speckle
decreases. If Z is increased, the size of the speckle increases. If Y
equals the focal length of the cylindrical lens, which is the smallest
aperture, the speckle will be larger than if the master diffuser 22 is off
the focal length in either direction.
The photosensitive medium 24 recorded with the vertically oriented
line-like speckles in the recording set-up of FIG. 7 can then be
replicated as described below and used as a directional homogenizer of the
present invention, or may itself be used as a master diffuser in another
recording set-up to achieve additional degrees of freedom as is seen in
FIG. 10. If the recorded photosensitive medium is used as a master
diffuser for subsequent recordings, it may be unnecessary to use lens 28
because the master diffuser will create the desired elliptical speckle in
the photosensitive medium 24.
Referring now to FIG. 10, there is depicted a coherent laser light source
18, objective lens 20, and a photosensitive medium 24 similar to those in
the previous drawings. Also depicted is a first master diffuser 32 and a
second master diffuser 34. The recording set-up in FIG. 10 is preferred
where the least amount of back scatter, the greatest amount of
transmission efficiency, and the greatest uniformity of intensity is
desired. By using two master diffusers prerecorded with, for example,
elliptical speckle oriented in the same direction in both master diffusers
32 and 34, elliptical speckle are generated in the photosensitive medium
24 which have better intensity than can be recorded with one master
diffuser. Furthermore, recording media of larger surface area are made
possible by using two master diffusers. Finally, a cylindrical lens need
not be used.
The output in FIG. 11A is rectangular and produced by recording in the same
photosensitive medium elongated elliptical speckle in the horizontal
direction and elongated elliptical speckle of a slightly lesser degree in
the vertical direction. These two recordings may be accomplished
sequentially using either volume holographic master diffusers prerecorded
with elliptical speckle or a cylindrical lens and a conventional round
output diffuser, or other combinations of lens and master diffusers. FIG.
11B shows an output having roughly the same full width half maximum (FWHM)
in the horizontal direction as in FIG. 11A but an FWHM in the vertical
direction reduced by roughly a factor of 2. Angular output was measured at
FWHM which is a measurement of angular spread of the output from the
homogenizer at all peripheral points which are at one-half the intensity
of light passing through the center of the directional homogenizer. The
sculpted surface features of the homogenizer having the output shown in
FIG. 11A is best described by two perpendicular, intersecting sets of
peaks and valleys in the surface as seen in the photograph in FIG. 11C.
The high efficiency of the homogenizers of the present invention is
highlighted by FIGS. 12A-E. Shown in FIG. 12A is a plot of power or light
intensity through a directional homogenizer versus the angular spread of
light output from the directional homogenizer in degrees. The homogenizer
of the present invention which produced the output shown in FIG. 12A is a
20.degree. circular homogenizer. In other words, this homogenizer has an
FWHM of approximately 20.degree. (19.55 actual). As can be seen from FIG.
12A, side lobe (the area outside the predefined output area or FWHM
illuminated area) intensity is minimal, thus conserving light energy.
Referring now to FIG. 12B a homogenizer having a FWHM of 10.degree. is
depicted. The minimal side lobes are especially apparent here where
intensity drops virtually to zero at about 12.degree. from center. This
homogenizer, as opposed to the homogenizer which produced the output of
FIG. 12A produces a very narrow circular spot of light. It can be
appreciated that homogenizers of an unlimited number of FWHM values may be
produced in accordance with the present invention, thus making possible
homogenizers having a myriad of output shapes and intensities suitable for
virtually any application.
FIGS. 12C, D, and E respectively show, for comparison purposes, the output
from a 20 micron ground glass diffuser, an acetate diffuser, and a
lenticular diffuser. It is apparent that the side lobes in each of FIGS.
12C-E are large which means that energy is wasted. Furthermore, it can be
appreciated that because of the manner in which ground glass, acetate, and
lenticular diffusers are produced, there is far less ability to control
irregularities in these diffusers and the exact output characteristics
desired. The present invention is a significant advance because it is easy
to create and control the speckle in the homogenizer to achieve the
desired homogenized pattern.
After recording, conventional development processes are used to develop the
photosensitive medium. In the case of DCG, water-alcohol baths are used to
swell the non-exposed areas to a greater degree than the exposed areas to
create the surface structure. If photoresist is used, the exposed areas
are removed and the unexposed areas remain intact when developed.
Once the photosensitive medium 24 is recorded and developed with the
desired features using any of the above recording set-ups or equivalent
ones, the photosensitive medium is processed as follows. The surface
structure of the photosensitive medium 24 may preferably be rendered into
a standard curable epoxy or silicone rubber or other molding agent. A
release agent is preferably applied to the photosensitive medium prior to
application of the epoxy to facilitate removal of the epoxy after curing.
It is preferable to evaporate a release layer on the angular spread such
as oil or another suitable "slippery" release agent. The epoxy may be
applied to the photosensitive medium and then a substrate, such as glass,
metal, or plastic, placed on top of the epoxy to sandwich it between the
photosensitive medium and the substrate. Alternatively, the epoxy may
first be applied to a substrate which has been roughened somewhat to
ensure that the epoxy sticks to it and then the epoxy sandwiched between
the photosensitive medium and the substrate.
It is necessary to ensure that the epoxy is uniformly sandwiched between
the photosensitive medium 24 and the substrate so that no air bubbles are
present. After the sandwich is completed, the epoxy is then cured under a
UV lamp, or cured after a passage of time if it is time curing epoxy, and
finally the epoxy is separated from the photosensitive medium. If the
photosensitive medium is DCG, additional epoxy replicas of it may be made,
called "parent" replicas.
Standard mass production techniques can be used to create large numbers of
exact copies of the parent (epoxy) replicas. Typically, the parent
replicas may be subjected to conventional electroform processes, such as
nickel electroform, to create a metal master which may then be used to
emboss polyester or any thermoformable plastic. The type of reproduction
used of course depends upon the number of copies desired and their
ultimate use.
In larger homogenizers, it is obviously preferable to make the surface area
of the photosensitive medium 24 as large as possible. In such a case, a
nickel electroform master may be used to emboss a plurality of
thermoformable plastic sheets which are then joined together to form a
larger surface.
Embodiments of the present invention not disclosed herein are fully
intended to be within the scope of the appended claims.
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